Electrode



Dec. 15 1942. R. D. LOWRY 2,305,539

ELECTRODE Filed May 23, 1938 Fig. 3 4

INVENTOR 20/5527 0. LOW/Q) WmMM ATTOZNE-VS tion.

Patented Dec. 15, 1942 ELECTRODE Robert D. Lowry, Midland, Mich, assignor to The Dow Chemical Company,

poration of Michigan Application May 23, 1938, Serial No. 209,535

4 Claims. The invention relates to articles of manufacture comprising a metal base having a magnetite coating. It more particularly concerns an article for use as an electrode in the electrolysis of aqueous solutions of alkali metal Midland, Mich, a cornoble metal in a magnetite base or surface suitably supported upon a metallic form, the embedhalides for the production of. chlorine, alkali hydroxides, and hydrogen and is susceptible. of use in many. other electrolytic processes.

It is highly desirable in the electrolysis of solutions tobring about the decomposition of the solution at a potential as near as possible to the theoretical decomposition potential of the solution, in order to effect decomposition with the least expediture of energy and therefore obtain the greatest economy of operation. Ordinarily in the electrolysis of solutions wherein some of the products of decomposition are gaseous, the phenomenon of over-voltage is observed and the potential which must be employed to cause a continuous liberation of the gaseous products from the surfaces of the electrodes is much higher than that theoretically necessary for such liberation I This over-voltage at the electrodes is the excess voltage or energy actually required to bring about continuous flow of current and decomposition of theelectrolyte over that necessary to satisfy the thermodynamic requirements for the formation of the products of decomposition. In some cases this excess voltage or the over-voltage necessary for decomposition of the solution may be equivalent to an energy loss of from 10 to 40 per cent-of the total energy required, and usually appears as heat dissipated into the electrolyte.

It has long been recognized that the nature of the electrodes determines to a great extent-the amount of the over-voltage in any electrolytic process. For example, it is well known that an electrode material which gives a very low overvoltage to either hydrogen (Hz) or chlorine (01:) when used in the electrolysis of sodium chloride brine is platinum covered with platinum black (called "platinized platinum), but this material is now generally prohibitively expensive for commercial use. At the two most commonly used electrode materials in electrolysis ofsalt solutions to make chlorine: and, while these materials are fairly satisfactory as regards current conductivity, they have the disadvantage in that the iron exhibits relatively high over-voltage characteristics, while the carbon is readily attacked by chlorine or oxygen.

I have now discovered that by embedding a and .continued decomposition of the solupresent carbon and iron are invention,

ding being performed in such a manner as to leave portions of the surface of both the magnetite and the noble metal exposed, the resulting composite electrode is highly resistant to corrosion and has desirable low over-voltage characteristics when employed in the electrolysis of alkali metal chloride solutions and the like.

In order to illustrate the invention reference is made to the accompanying drawing. In said drawing:

Fig. 1 is a side elevation in cross-section showing one construction of the new article.

Fig. 2 is an enlarged fragmentary sectional view showing the surface of the article in more detail.

Fig. 3 is a side elevation in cross-section depicting a slightly modified form of construction from that illustrated in Fig. l.

Fig. 4 is an enlarged fragmentary sectional view of the surface shown in Fig. 3 illustrating the surface-structure in more detail.

As shown in Fig. 1 the finished article forming the object, of this invention comprises a metal base I with a coating of .magnetite 2 covering the metal base. A suitable noble metal I is embedded in the magnetite so as to form a discontinuous exposed surface of both noble metal and magnetite. The manner of producing such discontinuous surface of exposed embedded noble metal and magnetite is hereinafter more fully described.

In Fig. 2 the surface of the article shown in Fig. l is shown in enlarged detail.

In the view illustrated in Fig. 3 the article of Fig. 1 is shown in slightly modified form. In .this case the metal base i has had formed thereon a discontinuous deposit ofa non-oxidizable metal 4. The non-oxidizable metal has in turn been coated with a noble metal 3. An adherent magnetite coating 2 is shown covering the surface of the metal base I in such a manner as to produce a discontinuous surface exposing both no-y ble metal 3 and magnetite, with the noble metal coated non-oxidizable metal I firmly embedded in the magnetite, 2.

In Fig, 4' the surface of thearticle shown'in Fig. 3 is shown in an enlarged detailed view.

The invention then consists of the article hereinafter fully described and particularly pointed out in the claims, the following description setting forth in detail certain embodiments of the illustrating, however. but several of 2 the various ways in'which the principle of the invention may be used.

Various modes of producing an electrode of the desired type may be employed. A preferred method of producing such an electrode is to subject a piece of iron or steel stock conforming in shape to that desired in the finished electrode to the following treatment: The shaped iron article is first sprayed with a metal of the noble metal class in such a manner as to form an even deposit on the iron article without completely covering it, thereby leaving small uncoated areas of the iron surface exposed between the'particles of the deposited metal. Such a deposit of the noble metal upon the ironsurface is produced by moving the nozzle of the spray gun relatively rapidly across coated. I have found that a deposit of the applied metal covering between about 20 to 80 per cent of the area of the iron surface is sufficient. In forming the deposit of the noble metal any well known method of spraying a molten metal in a stream of high velocity gas may be used, such as the Schoopmetal spray method. The metal particles, being carried in the gas stream while still in a molten or near molten state, strike the iron surface with considerable force and become mechanically bonded to the iron surface, forming a matte-like deposit not completely hiding the iron. If desired, the iron may be preheated to a temperature of from 200 to 300 C, or more to facilitate the formation of the bond between the metals.

After the shaped article has been spray coated with the noble metal the' exposed iron surface is converted to magnetite. The preferred method of bringing about the conversion of the iron surface to magnetite is by subjecting the iron to the action of high temperature steam. For example, iron subiected to the action of steam at a temperature of from between about 600 to 900 C. is readily converted to magnetite. The magnetite first appears as a thin surface film on the iron and gradually, under continued action of the steam, further oxidation of the iron to magnetite takes place until eventually a magnetite coating of considerable thickness can be built up. 'It has been found that a satisfactory magnetite coating of the type which will strongly bond to and hold the particles of the applied metal coating may be produced in about 5 hours in the presence of steam at a temperature of approximately 850 C., or in about hours at a temperature of approximately 650 C. The surface being converted to magnetite should not be the action of steam for an excessive length of time, because prolonged action of the high tem-' perature steam may cause the magnetite to build up around the noble metal particles to the extent that they may be completely covered by the magnetite and the desirable low over-voltage effect due to the presence of such metal on the surface of the magnetite will be lost. As the iron surface is converted, the magnetite builds up or grows around the noble metal particles producing a magnetite surface with particles of the noble metal having low over-voltages characteristics embodied therein. Since the particles of noble metal embedded in magnetite are highly irregular in shape and surface contour, an effective electrode surface of relatively great area and having low over-voltage characteristics is formed by use of a comparatively small amount of the nobel metal. The composite electrode thus obtained is not, only highly resistant to corrosion under the conditions of inthe surface of the article to be subjected to and passing a suitable tended use, but the particles of metal having low over-voltage characteristics are strongly held in the magnetite coating and cannot spall of when in use, as is the case if the article is not magnetited after depositing the noble metal. It is to be understood that the shaped article on which the noble metal is deposited and converted at least superficially to magnetite may consist entirely of iron or steel, or if desired, it may be made from nickel, copper or other metals and coated with iron.

A similar electrode having high corrosion resistance and low over-voltage characteristics may be made by first depositing, as by spray coating, a porous coating of a substantially non-oxidizable metalor alloy, such as Nichrome or Monel, on an iron or iron coated metal base in amount sufficient to cover 20 to per cent of the surface area of the article, electroplating a coating of a noble metal on the non-oxidizable metal without depositing the noble metal on the exposed iron or steel surface, and converting the exposed iron surface to magnetite. The electro-deposition of the noble metal on the non-oxidizable metal is accomplished by immersing the article in an electrolyte solution containing a. noble metal ion electric current between the article which acts as one electrode and another electrode immersed in the solution. Since the noble metal plates out upon the non-oxidizable metal much more readily than upon the iron surface, only the non-oxidizable metal is coated with the noble metal. If desired the step of converting the iron to magnetite may be carried out before the noble metal is electro-deposited on the substantially non-oxidizable metal and the noble metal electro-plated upon the exposed nonoxidizable metal after the magnetiting operation. Still another method of forming an electrode of similar type consists in depositing upon an iron surface a porous coating of a metal that is relatively readily oxidized by superheated steam, such as copper, electroplating a low over-voltage metal onto the oxidizable metal and converting the exposed iron to magnetite. The magnetite builds up around the oxidizable metal and eventually only noble metal and magnetite are exposed.

If desired, a suitable electrical connection, such as a conventional connecting lug, may be. welded onto the base metal before the article is magnetited, since it is difflcult to make a suitable electrical connection directly to magnetite. The connection lug may be suitably made from nickel, for example, since this metal is not changed appreciably under the conditions which convert iron into magnetite.

The noble metals that may be used are any of the metals of the noble metal group, such as platinum, rhodium, palladium, iridium, silver, gold, platinum-rhodium alloys, and other alloys of these metals. In general platinum and platinum-rhodium alloys are the most satisfactory.

The following example is illustrative of a preferred mode of carrying out the invention.

A piece of iron conforming in shape to the desired shape of the finished electrode and havin welded thereto a nickel bar to act as an electrical connection, was cleaned thoroughly by sand blasting. The cleaned article was then preheated to a temperature of from 200300 C. and platinum sprayed on the heated amount used being approximately .3 lb. per sq. ft. of surface area. The spray coated article was then placed in a well thermally insulated reaction chamber and subjected to the action of iron surface, the

superheated steam at a temperature of 650 C. for 15 hours. The steam was then shut 08 and the electrode allowed to cool slowly. After cooling the electrode was ready for use. The electrode so formed when used as an anode in the electrolysis of aqueous sodium chloride solution and compared with a graphite anode under the same conditions of use had a lower chlorine over-voltage than the graphite anode, and after continued use little or no corrosion of the magnetite-noble metal surface could be observed, while the graphite anode was observed to be extensively corroded.

Other modes of applying the principle of my invention may be employed instead of those explained, change being made as regards the method herein disclosed or the materials employed, provided the step or steps stated by any of the following claims be employed or the product claimed in any of the following claims be obtained.

I therefore particularly point out and distinctly claim as my'inven-tion:

1. Acomposite article of manufacture comprising a metal base having .an adherent layer of magnetite thereon in which is embedded a discontinuous deposit of a noble metal so as to leave both magnetite and noble metal exposed, said coating of magnetite being of such a thickness as to support and firmly hold the deposit of noble metal in place.

2. A composite article of manufacture comprising an iron base having an adherent layer of magnetite thereon in which is embedded a discontinuous deposit of a metal selected from the noble metal class of metals so as to leave both magnetite and noble metal exposed, said magnetite being of such a thickness as to support and firmly hold the deposit of noble metal in place.

3. A composite article of manufacture comprising a metal base having an adherent layer of magnetite thereon in which is embedded particles of a substantially non-oxidizable metal, said nonoxidizable metal having a surface coating of a noble metal, the said magnetite coating being of sufiicient thickness to support and firmly hold the particles of noble metal coated non-oxidize ablemetal in place.

' 4. A composite article of manufacture comprising a metal base having an adherent layer of magnetite thereon, and having embedded in the surface of the magnetite layer particles of a noble metal selected from the group consisting of platinum, rhodium, iridium, palladium, silver, and gold so as to leave both the magnetite and the embedded metal exposed, said magnetite being of sumcient thickness to support and firmly hold the particles of noble metal in place.

ROBERT D. LOWRY. 

